This invention relates to sputtering apparatus and more particularly to improvements in planar cathode sputtering devices.
Sputtering is a well known method of providing relatively uniform deposits of material on a substrate. Essentially, sputtering is accomplished by the impingement of gas ions on a cathode plate or target plate in the presence of an electrostatic field which causes particles of the cathode plate material to be dislodged therefrom. By appropriately positioning a substrate in the path of such dislodged particles, a deposit of cathodic material will be produced on the substrate. While sputtering enables relatively uniform deposition on a substrate, the rate of deposition has been severely limited. Consequently, sputtering techniques have been limited to those applications wherein thin films, on the order of several micrometers, are to be deposited on a substrate.
Recently, increased sputtering rates have been achieved by confining the glow discharge plasma, in which the gas ions are produced, in close proximity to the target plate, such that ionization of the gas will occur more frequently. In this manner, an increased rate of ion impingement upon a target plate, and hence greater sputtering activity, is enabled. More specifically, a magnetic field is provided such that flux lines extend from and return in an arched or curved path to the surface of a target plate thereby forming a virtual "tunnel". By providing the aforementioned magnetic field in a closed loop or "racetrack" configuration, electrons will tend to be swept about this loop under the influence of the applied magnetic field and electric field. It is the resulting plasma confinement which promotes a high degree of ionization of molecules of the ambient gas, e.g. argon. The ions are electrostatically attracted to the target plate and thus effectively stimulate a high degree of sputtering activity and correspondingly high deposition rates of cathodic, or sputtered, material on a substrate. Typical of the aforedescribed sputtering apparatus is the device illustrated in U.S. application Ser. No. 438,482, filed on Jan. 31, 1974, in the name of John S. Chapin.
It has been found that material sputtered from a target plate subjected to the aforedescribed magnetic field is dislodged primarily from an erosion region underlying the curved flux lines. In addition, it has been found that a region or location exists at which such flux lines are parallel to the cathode. This, in turn has resulted in maximum target erosion at a region substantially aligned with and underlying the foregoing point or area over which magnetic flux lines are parallel to the target plate. When a complete target plate area is considered, it has been found that a line defining the nadir of a deep "valley" exists in the cathode plate approximately centrally of the aforementioned closed magnetic loop. Accordingly, deep and acute erosion of a cathode plate will readily extend completely therethrough and although a major portion of the target plate remains available as a source of sputtering material, the sputtering rate will decrease due to the absence of target material underlying the location of parallel magnetic flux lines. Tests have shown that under the foregoing circumstances, only approximately 38% by weight of the target plate is actually realized as sputtered material. Accordingly, a clear need exists for methods and apparatus capable of increasing the amount of material which may be sputtered from a planar cathode before the target plate must be replaced, particularly in view of the ever increasing costs of commonly sputtered materials such as copper, palladium, titanium, chromium, etc.